Distribution of trace elements and minerals in human and cow's milk

The concentration of iron in cow's milk, 0.40-0.59 μg/ml, was found to be very similar to that of human milk, 0.20-0.69 μg/ml. The copper concentration of cow's milk (0.06-0.09 μg/ml) is lower than in human milk (0.24-0.50 μg/ml) whereas the concentration of zinc is higher in cow's milk (3.23-5.15 μg/ml) than in human milk (1.16-3.83 μg/ml). Cow's milk contains about 4-5 times more calcium and magnesium, 854-1430 μg/ml and 87-131 μg/ml, respectively, than human milk (220-252 μg/ml and 26-35 μg/ml). Cow's milk was fractionated and the trace element and mineral contents of the different fractions were compared to results from human milk. The casein fraction in cow's milk contains a large proportion of the total amounts of the elements cited above (Fe 24%, Cu 44%, Zn 84%, Ca 41%, Mg 25%) whereas human casein only binds minor amounts (Fe 9%, Cu 7%, Zn 8%, Ca 6%, Mg 6%). Whey proteins bind a major part of these elements in human milk, but not in cow's milk. Significant amounts of iron are bound to the lipid fraction in both cow's and human milk (14 and 33%, respectively), predominantly bound to the outer fat globule membrane. Low molecular weight compounds (ligands) bind significant proportions of all the elements investigated in both cow's and human milk, with the exception of zinc in cow's milk, of which only 2% is associated with this fraction. The differences in trace element bioavailability observed in infants fed human milk versus cow's milk (formula) may therefore be explained, in part, by differences in ligand binding; the differences in distribution between the casein and whey proteins may be especially important. It is possible that casein may act as a limiting factor for the absorption of trace elements and minerals in the newborn infant with limited digestive function.